(1) Field of the Invention
This invention relates to applying a media access based on carrier sense multiple access/collision detection (hereinafter referred to as CSMA/CD), which is generally used in a bus network, to a star network.
(2) Description of the Related Art
Home Bus System (HBS) is one of the systems employing a survival media access based on CSMA/CD.
FIG. 1(a)/(b)/(c) respectively depict a frame configuration, a character code configuration, and how each frame is transmitted in such a system.
As shown in FIG. 1(a), one frame consists of blocks of character codes, namely, a PRiority code (PR), a Source Address (SA), a Destination Address (DA), a Control Code (CC), a Byte Count (BC), DATA, an Error Check Code(ECC), a DuMmY code (DMY), and ACKnowledge/Not AcKnowledge (ACK/NAK). The first two codes, PR and SA have a function as a bit string to determine a survival in contention control. DATA is variable in length up to a maximum of 256 characters, the other codes having fixed lengths.
As shown in FIG. 1(b), one character code consists of an 8-bit data unit, start and stop bits for start-stop transmission, and a parity bit for error detection.
As shown in FIG. 1(c), continually monitoring a signal on the bus, each terminal device counts a pause of 10 mS every time the last transmission of a packet from any terminal device including itself is completed, and then starts the transmission of its own packet, if any. The pause includes a synchronous recovery monitoring period corresponding to the last 2 bits of the entire length thereof. If any terminal device starts to transmit a packet within this period due to its pause-counting error, the others can follow without waiting for the end of the pause. Thus, each terminal device can fairly use the bus.
FIG. 2 explains how terminal device 1 wins in the competition and continues the transmission when terminal devices 1 and 2 have tried to access at the same instant. FIG. 2(a) shows PR and SA of a packet sent from terminal device 1, while FIG. 2(b) shows the same sent from terminal device 2, both PRs being identical. Their waveforms are respectively shown in FIG. 2 (c)/(d). FIG. 2(e) depicts the waveform of the signal on a bus which is the result of the codes sent from each terminal device being wired-ORed.
When terminal devices 1 and 2 are in contention, the waveform of PR of a signal on the bus coincides with the codes of both terminal devices 1 and 2. On the other hand, the waveform of SA of the signal on the bus becomes logical 1 as the result of logical 1 from terminal device 1 and logical 0 from terminal device 2 being wired-ORed.
Monitoring the waveform of a signal on the bus per bit, terminal device 2 suspends transmission as soon as it detects discordance between its own code (logical 0) and the waveform (logical 1) of the signal on the bus as shown in FIG. 2. Terminal device 1, on the other hand, continues the transmission to the end without noticing the collision. Since the codes in SAs on the terminal devices are all different from each other, any one of the terminal devices can surely transmit a packet.
Such HBS used in a bus network has been expected to be applied to a distributed data-processing star network by utilizing infrared radiation as its transmission media.
In some systems using infrared radiation for transmitting control signals such as remote control signals or information signals such as voices, images, and data, allocation of sub-carrier frequency has been proposed to avoid cross-interference between signal channels ("Sub-carrier frequency allocation for infrared data transmission" EIAJ CP-1205) and ("Infrared Applications" IEC TC84/WG17).
FIG. 3 shows an example of channel allocation of sub carriers. Channel 1 is allocated a frequency band for existing remote control, Channel 2 is allocated another frequency band for convention system and analog voice transmission, whose use is considered difficult in Japan because of the affects of luminaire with high frequency lighting. Channel 3, 4 and 5 are respectively allocated frequency bands for data transmission, for hi-fi voice transmission, and image transmission.
However, a survival media access based on CSMA/CD of a bus network has been unable to be applied to a star network. For example, a data signal being transmitted through channel 3 must be first modulated with a sub-carrier having a sine wave between 1 and 2 MHz; however, in the survival media access based on CSMA/CD, the modulated signal may collide at any phase, causing phase distortion or reverse phase. In other words, detected logical levels may become indefinite or garbled, which may hinder proper contention control or proper detection of a start bit for start-stop transmission.